This invention relates to thyristors and more specifically relates to a novel process for balancing the forward and reverse characteristics of a thyristor.
Thyristors, sometimes called silicon controlled rectifiers or SCRs, are devices of well-known construction and characteristics. Very high voltage thyristors, for example those having a blocking voltage of 1,200 volts or greater, commonly have a bevel around their outer surface which enables junctions reaching the outer periphery of the chip to withstand the high blocking voltage by spreading out the electric field at the junction edge. Commonly, a double bevel is used so that the upper junction of the device has a smaller interior bevel angle than the lower junction. Commonly, the uppermost emitter junction is a planar junction terminating on the upper surface of the wafer in which the device is formed.
It is desirable that reverse blocking voltage be equal to the forward blocking voltage at room temperature, in such devices, and further that the reverse leakage current be as close as possible to the forward leakage current, at high temperatures. Commonly, however, the forward blocking voltage is less than the reverse blocking voltage and the forward leakage current is greater than the reverse leakage current. One reason for these differences is the difference in the alphas of the PNP transistor section and the NPN transistor section of the thyristor.
It is known that thyristor characteristics can be controlled by proton, alpha particle, or electron irradiation of the thyristor. The use of nuclear irradiation for reducing the switching time of a thyristor is disclosed in U.S. Pat. No. 4,056,408 to Bartko and Sun, issued Nov. 1, 1977. Bartko, however, does not disclose a process for modifying and balancing the forward and reverse blocking voltage and the forward and reverse leakage currents of a thyristor.